How the exposure calculator works
The stop as the universal currency
One stop = doubling (or halving) light. Shutter: 1/500 → 1/250 is +1 stop. Aperture: f/2.8 → f/4 is −2 stops (area scales with the SQUARE of the f-number — hence the squared term). ISO: 400 → 800 is +1 stop of amplification. Every exposure decision is moving stops between three dials.
The side effects are the real decision
Equivalent exposures produce identical brightness and different photos: slower shutter adds motion blur, wider aperture narrows focus, higher ISO adds noise. The photographer's skill is choosing WHICH side effect the shot needs — the calculator handles the arithmetic so the choice is deliberate.
The standard scales
Shutters: 8000, 4000, … 500, 250, 125, 60, 30, 15, 8, 4, 1″. Apertures: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 — each jump is one stop (the √2 progression). ISO: 100, 200, 400, 800… Learning the scales by heart makes the triangle mental arithmetic; the calculator fills the gaps meanwhile.